1. Field of the Invention
The present invention relates to a package for hermetically sealing electronic circuits.
2. Description of the Prior Art
Packages for enclosing electronic circuits, such as hybrid circuit devices and large scale integrated (LSI) wafers, are well known. Hybrid packages for electronic circuits commonly comprise a square or rectangular ceramic-glass-metal construction in which glass layers are used to bond a metal ring frame to a ceramic base and to insulate the frame electrically from leads passing thereunder. A variation of this design utilizes a ceramic ring frame in which the top surface thereof is metallized for soldering. Large scale integrated wafers are generally placed within a circular package having a radial pattern of metallized leads which are overcoated with a glass or ceramic dielectric which, in turn, is overcoated with a solderable metallizing material. Use of glass in a ceramic-metal seal invariably results in a brittle seal which is susceptible to failure from mechanical and thermal shocks.
Package lids for both hybrid electronic circuits and large scale integrated wafers are commonly made of sheet metal which is drawn into a shallow square, rectangular or circular pan. To seal the package, the lip of the lid is butt soldered to a flat sealing area formed at the perimeter of the package. In order to assure adequate hermeticity for long term storage and reliable performance, a smooth solder fillet is formed around the outside of the butt joint.
Some of the problems heretofore encountered with this type of lid and seal include low strength and reliability of a butt solder joint. Although solder tensile strengths typically measure in the range of 3,000 psi to 7,000 psi, under long-term fatigue conditions, solder strength can degrade to 1/10th of its original value. When a package with a metal pan-shaped lid is subjected to atmospheric pressure fluctuations, as may be encountered in an electronic system which is repeatedly carried aloft to high altitudes in an airplane or submerged to great depths in a submarine, the surface of the metal lid deflects appreciably. Such deflections exert cantilever moments on the solder joint and cause seal failure through fatigue in the solder. Reduced atmospheric pressure conditions cause excessive tensile forces to be exerted on the butt solder joint.
Although lid deflection can be reduced by increasing the thickness of the lid, the weight of the package is also increased, which may be an objectionable or an intolerable weight increase, and the thicker metal wall at the butt joint also becomes more difficult to solder. Furthermore, in order to match more closely the thermal expansion of the ceramic base of the hybrid package, the lid is made of an iron-nickel-cobalt alloy marketed under such trademarks as "KOVAR", "THERLO" and "FERNICO". Such alloys are magnetic, which can produce electrical problems with some types of circuitry installed in the electronic package, although use of such alloys to provide magnetic shielding at times is desirable. Nevertheless, conventional seal and lid designs utilizing this alloy for the lid generally create serious problems which affect performance and reliability of the packages.